There is routinely carried out a clinical test which comprises forming hydrogen peroxide by using an oxidase from a component to be measured in a biological specimen such as serum and measuring the hydrogen peroxide formed to thereby determine the component to be measured. Especially, colorimetric analysis is routinely used in the clinical test. In the colorimetric analysis, the formed hydrogen peroxide is reacted with an oxidative-coloring chromogen in the presence of a peroxidase to form a dye, and the absorbance of the colored reaction solution containing the formed dye is measured to thereby determine the component to be measured in the biological specimen.
In this colorimetric analysis based on measuring hydrogen peroxide, the influence of interfering substances such as ascorbic acid and bilirubin contained in biological specimens often becomes a problem. Especially, as to bilirubin, there is a problem that bilirubin influences the reaction of hydrogen peroxide with an oxidative-coloring chromogen in the presence of a peroxidase, and its reduction action gives a negative influence.
Methods known so far to solve this problem include a method of using a ferrocyanide ion (see patent document 1), a method of using an EDTA-iron complex (see patent document 2), a method of using a ferrocyanide ion and albumin (see patent document 3), a method of using a cationic surfactant and/or an amphoteric surfactant (see patent document 4), a method of using an amphoteric surfactant (see patent document 5), a method of using an amphoteric surfactant and a ferrocyanide (see patent document 6), a method of using a polyoxyethylene alkyl ether having an HLB of 13 or more and a ferrocyanide ion (see patent document 7), a method of using an iron complex and a steroid compound (see patent document 8), and a method of using a polyoxyethylene alkylphenyl ether condensate (see patent document 9).